Go forbids import cycles. How do you detect and break them?

The compiler rejects cycles (import cycle not allowed) because package initialization and compilation units must form a DAG. A cycle usually signals a design problem: two packages that are really one, or a dependency pointing the wrong way.

Ways to break them:

1. Dependency inversion with consumer-side interfaces. The lower-level package defines the small interface it needs, and the higher-level package satisfies it implicitly.
2. Extract shared types into a leaf package (for example domain or model) that both import. Avoid turning it into a dumping-ground util package.
3. Merge the packages if they are really one unit.
4. Wire dependencies in main (composition root): pass funcs or interfaces in at construction time.

// package order (no import of package user)
type UserLookup interface {
    Email(ctx context.Context, id int64) (string, error)
}
type Service struct{ users UserLookup }

// package user imports order freely; main wires:
//   svc := order.Service{users: userRepo}

Tests: an external test package (package order_test) can import packages that import order, which breaks test-only cycles. Diagnose with go list -deps or go mod graph at the module level. Interviewers look for "accept interfaces, defined where they are consumed" instead of hacks such as moving code into main.